feat: add immutable cognitive event store, live streams and event explorer

Phase 3. Events are the append-only record of everything a mind does
(ADR-0002). pkg/events defines the closed type vocabulary, categories,
evidence references and the observation payload with explicit trust
levels. The store validates and appends inside the caller's transaction,
publishes to a bounded fan-out bus only after commit, queries by seq
cursor with type/category/source/salience/confidence/time/search
filters, and walks provenance: cause, parents, children, cited evidence,
derived events and correlation groups. Durable per-consumer cursors never
move backwards.

Server-Sent Events streams (per mind and global) subscribe before
replaying history after Last-Event-ID so nothing is missed, apply the
same filter vocabulary live, and heartbeat. Operators can inject
observations tagged with operator trust for demos and tests.

UI: live thought stream with filters and paging, forensic event page
with causal chain and evidence navigation, dashboard important events.
Personality and status changes now leave operator_action events in the
mind's own history. CLI: event inspect/list/tail, observe.

web/templates/partials/mind_tabs.html

@@ -0,0 +1,8 @@
+{{define "mind_tabs"}}{{$m := .M}}{{$a := .Active}}
+<nav class="tabs" aria-label="Mind sections">
+  <a href="/minds/{{$m.Slug}}" {{if eq $a "overview"}}aria-current="page"{{end}}>Overview</a>
+  <a href="/minds/{{$m.Slug}}/thoughts" {{if eq $a "thoughts"}}aria-current="page"{{end}}>Thoughts</a>
+  <a href="/minds/{{$m.Slug}}/personality" {{if eq $a "personality"}}aria-current="page"{{end}}>Personality</a>
+  <a class="disabled" title="Phase 4">Workspace</a><a class="disabled" title="Phase 5">Memory</a><a class="disabled" title="Phase 6">Beliefs</a><a class="disabled" title="Phase 8">Conversation</a>
+</nav>
+{{end}}